The present invention relates to a system for automatically controlling weld material beading in automatic orbital welding processes for medium- and large-size pipes, such as pipes for oil and gas pipelines and the like.
It is known that the method for performing orbital welding with the known processes termed "MIG" or "MAG" (Metal Inert Gas or Metal Active Gas) consists in depositing, by means of multiple runs, a series of overlapping welding beads in the flared groove, commonly termed welding gap, which is delimited by chamfers or flared regions that are provided on the juxtaposed heads of the sequentially arranged pipe segments. The apparatus for performing this welding process is substantially constituted by an annular rail that is fitted and locked on the pipeline and by one or more movable welding carriages, typically two, which move at a controlled speed and in opposite directions along said rail, each one orbiting around the pipeline over a 180.degree. arc. Each carriage has wheels for engaging the rail and sliding on it, a driving pinion meshing with a toothed rim of the rail to move the carriage, and at least one oscillating welding torch of the continuous-wire type.
During the welding process, the rail is placed proximate to the section to be welded, and the welding carriages are engaged on it; the welding torches of the carriages are aligned with said welding gap. Owing to the flared profile of the welding gap, the amount of deposited material increases gradually as the torch moves away from the axis of the pipe with each run. Accordingly, the torch, which is initially fixed with respect to the carriage, is subjected to an oscillating motion the extent, frequency, and turnaround period (stop time after each elongation) whereof vary, both in passing from one welding bead to the next and in forming a same welding bead, according to the position of the carriage along the circumference of the pipe, due to the influence of gravity on the deposited metal, which is liquid. The motion speed of the carriage and the rate at which the welding wire is fed also depend on the position of the carriage along the circumference of the pipe, and in current orbital welding devices this set of parameters is controlled by a microprocessor according to programmed sequences for each run and for a given welding gap profile.
It is evident that correct automatic control of the many welding parameters assumes that said welding gap profile is constant; however, said profile can undergo even significant variations due to imperfections in machining, which is usually performed on site, due to more or less conspicuous axial misalignments of the juxtaposed pipe segments, due to the imperfect linearity of the pipe segments, and/or due to the imperfect perpendicularity of the head sections with respect to the axis of the pipeline being treated. Any unevennesses in the material of the pipes, such as hollows, cracks, and the like, can also affect the profile of the welding gap, whose axis may furthermore lie at least partially outside of the ideal perpendicular cross-section of the pipe and have, entirely or partially, a helical orientation with respect to the axis of the pipe.
The unevennesses of the profile and of the axial orientation of the welding gap are, in any case, unavoidable and cause considerable problems in the above mentioned automated orbital welding processes, often forcing to resort to manual interventions of specialized workers, who monitor the welding process and intervene, by means of appropriate remote controls, so as to compensate the unevennesses of the welding gap by varying one or more of the parameters of the welding process.
Thus, for example, a discontinuity constituted by a localized tapering of the profile of the welding gap at the level of the current run is compensated manually by reducing the extent of the oscillation and the turnaround period of the torch and/or the rate at which the welding wire is fed, in order to momentarily reduce the weld material beading so as to keep the thickness of the welding bead constant along the entire circumference of the pipe.
On the contrary, a localized widening of the profile of the welding gap at the level of the current run requires an increase in the extent of the oscillation of the torch, an increase in the rate at which the wire is fed, and a possible decrease of the oscillation turnaround period, in order to momentarily increase the weld material beading, again to keep the thickness of the welding bead constant along the entire circumference of the pipe. If instead, the orientation of the axis of the welding gap has discontinuous portions that have a helical orientation, it is necessary to intervene so as to correspondingly move the oscillation axis of the torch, so as to prevent the oscillation itself from being off-center with respect to the welding gap and prevent the welding wire from interfering with one of the walls of said gap.